The data on sex, which was asked of all people, were derived from answers to long-form questionnaire Item 3 and short-form questionnaire Item 5. Individuals were asked to mark either "male" or "female" to indicate their sex. For most cases in which sex was not reported, it was determined from the persons given (i.e., first) name and household relationship. Otherwise, sex was imputed according to the relationship to the householder and the age of the person. (For more information on imputation, see "Accuracy of the Data.")

The industry classification system used during Census 2000 was developed for the census and consists of 265 categories for employed people, classified into 14 major industry groups. From 1940 through 1990, the industrial classification has been based on the Standard Industrial Classification (SIC) Manual . The Census 2000 classification was developed from the 1997 North American Industry Classification System (NAICS) published by the Office of Management and Budget, Executive Office of the President. NAICS is an industry description system that groups establishments into industries based on the activities in which they are primarily engaged.

The NAICS differs from most industry classifications because it is a supply-based, or production-oriented economic concept. Census data, which were collected from households, differ in detail and nature from those obtained from establishment surveys. Therefore, the census classification system, while defined in NAICS terms, cannot reflect the full detail in all categories. NAICS shows a more detailed hierarchical structure than that used for Census 2000. The expansion from 11 divisions in the SIC to 20 sectors in the NAICS provides groupings that are meaningful and useful for economic analysis. Various statistical programs that previously sampled or published at the SIC levels face problems with the coverage for 20 sectors instead of 11 divisions. These programs requested an alternative aggregation structure for production purposes which was approved and issued by the Office of Management and Budget on May 15, 2001, in the clarification Memorandum No. 2, "NAICS Alternate Aggregation Structure for Use by U.S. Statistical Agencies." Several census data products will use the alternative aggregation, while others, such as Summary File 3 and Summary File 4, will use more detail.

The data on class of worker were derived from answers to long-form questionnaire Item 29. The information on class of worker refers to the same job as a respondents industry and occupation, categorizing people according to the type of ownership of the employing organization. The 'class of worker' categories are defined as follows:

Private wage and salary workers include people who worked for wages, salary, commission, tips, pay-in-kind, or piece rates for a private for-profit employer or a private not-for-profit, tax-exempt, or charitable organization. Self-employed people whose business was incorporated are included with private wage and salary workers because they are paid employees of their own companies. Some tabulations present data separately for these subcategories: "for-profit," "not-for-profit," and "own business incorporated."

Government workers includes people who were employees of any federal, tribal, state, or local governmental unit, regardless of the activity of the particular agency. For some tabulations, the data were presented separately for federal (includes tribal), state, and local governments. Employees of foreign governments, the United Nations, or other formal international organizations were classified as "federal government," unlike the 1990 census when they were classified as "private not-for-profit."

In tabulations, this category is included with private wage and salary workers because they are paid employees of their own companies.

The industry category, "Public administration," is limited to regular government functions, such as legislative, judicial, administrative, and regulatory activities of governments. Other government organizations, such as schools, hospitals, liquor stores, and bus lines, are classified by industry according to the activity in which they are engaged. On the other hand, the 'class of worker government' categories include all government workers.

In some cases, respondents supplied industry, occupation, or class of worker descriptions that were not sufficiently specific for a precise classification or did not report on these items at all. In the coding operation, certain types of incomplete entries were corrected using the Alphabetical Index of Industries and Occupations . For example, it was possible in certain situations to assign an industry code based on the occupation reported, or vice versa.

Following the coding operations, there was a computer edit and an allocation process. The edit first determined whether a respondent was in the universe that required an industry and occupation code. The codes for the three items (industry, occupation, and class of worker) were checked to ensure they were valid and were edited for their relation to each other. Invalid and inconsistent codes were either blanked or changed to a consistent code.

If one or more of the three codes was blank after the edit, a code was assigned from a "similar" person based on other items, such as age, sex, education, farm or nonfarm residence, and weeks worked. If all of the labor force and income data were blank, all of these economic items were assigned from one other person or one other household who provided all the necessary data.

Comparability of industry and occupation data was affected by a number of factors, primarily the systems used to classify the questionnaire responses. For both the industry and occupation classification systems, the basic structures were generally the same from 1940 to 1970, but changes in the individual categories limited comparability of the data from one census to another. These changes were needed to recognize the "birth" of new industries and occupations, the "death" of others, the growth and decline in existing industries and occupations, and the desire of analysts and other users for more detail in the presentation of the data. Probably the greatest cause of noncomparability is the movement of a segment of a category to a different category in the next census. Changes in the nature of jobs and respondent terminology and refinement of category composition made these movements necessary. The 1990 occupational classification system was essentially the same as the 1980 census. However, the industry classification had minor changes between 1980 and 1990 that reflected changes to the Standard Industrial Classification (SIC).

In Census 2000, both the industry and occupation classifications had major revisions to reflect changes to the North American Industrial Classification System (NAICS) and the Standard Occupational Classification (SOC). The conversion of the census classifications in 2000 means that the 2000 classification systems are not comparable to the classifications used in the 1990 census and earlier.

Other factors that affected data comparability over the decades include the universe to which the data referred (in 1970, the age cutoff for labor force was changed from 14 years old to 16 years old); the wording of the industry and occupation questions on the questionnaire (for example, important changes were made in 1970); improvements in the coding procedures (the Employer Name List technique was introduced in 1960); and how the "not reported" cases were handled. Prior to 1970, they were placed in the residual categories, "industry not reported" and "occupation not reported." In 1970, an allocation process was introduced that assigned these cases to major groups. In Census 2000, as in 1980 and 1990, the "not reported" cases were assigned to individual categories. Therefore, the 1980, 1990, and Census 2000 data for individual categories include some numbers of people who would have been tabulated in a "not reported" category in previous censuses.

The following publications contain information on the various factors affecting comparability and are particularly useful for understanding differences in the occupation and industry information from earlier censuses: U.S. Census Bureau, Changes Between the 1950 and 1960 Occupation and Industry Classifications With Detailed Adjustments of 1950 Data to the 1960 Classifications, Technical Paper No. 18, 1968; U.S. Census Bureau, 1970 Occupation and Industry Classification Systems in Terms of Their 1960 Occupation and Industry Elements, Technical Paper No. 26, 1972; and U.S. Census Bureau, The Relationship Between the 1970 and 1980 Industry and Occupation Classification Systems, Technical Paper No. 59, 1988. For citations for earlier census years, see the 1980 Census of Population report, PC80-1-D, Detailed Population Characteristics. The 1990 census introduced an additional class of worker category for "private not-for-profit" employers, which is also used for Census 2000. This category is a subset of the 1980 category "employee of private employer" so there is no comparable data before 1990. Also in 1990, employees of foreign governments, the United Nations, etc., were classified as "private not-for-profit," rather than "Federal Government" as in 1970, 1980, and Census 2000. While in theory, there was a change in comparability, in practice, the small number of U.S. residents working for foreign governments made this change negligible.

Comparability between the statistics on industry and occupation from Census 2000 and statistics from other sources is affected by many of the factors described in the "Employment Status" section. These factors are primarily geographic differences between residence and place of work, different dates of reference, and differences in counts because of dual job holdings. Industry data from population censuses cover all industries and all kinds of workers, whereas, data from establishments often exclude private household workers, government workers, and the self employed. Also, the replies from household respondents may have differed in detail and nature from those obtained from establishments.

Occupation data from the census and data from government licensing agencies, professional associations, trade unions, etc., may not be as comparable as expected. Organizational listings often include people not in the labor force or people devoting all or most of their time to another occupation; or the same person may be included in two or more different listings. In addition, relatively few organizations, except for those requiring licensing, attained complete coverage of membership in a particular occupational field.